A traditional approach to produce a neutral beam from a negative ion H−, D− beam for plasma heating or neutral beam assisted diagnostics, is to neutralize the negative ion beam in a gas or plasma target for detachment of the excess electrons. However, this approach has a significant limitation on efficiency. At present, for example, for designed heating injectors with a 1 MeV beam [R. Hemsworth et al., 2009, Nucl. Fusion 49 045006], the neutralization efficiency in the gas and plasma targets will be about 60% and 85%, respectively [G. I. Dimov et al., 1975, Nucl. Fusion 15, 551], which considerably affects the overall efficiency of the injectors. In addition, the application of such neutralizers is associated with complications, including the deterioration of vacuum conditions due to gas puffing and the appearance of positive ions in the atomic beam, which can be significant in some applications.
Photodetachment of an electron from high-energy negative ions is an attractive method of beam neutralization. Such method does not require a gas or plasma puffing into the neutralizer vessel, it does not produce positive ions, and it assists with beam cleaning of fractions of impurities due to negative ions. The photodetachment of an electron corresponds to the following process: H−+hω=H0+e. Similar to most negative ions, the H− ion has a single stable state. Nevertheless, photodetachment is possible from an excited state. The photodetachment cross section is well known [see, e.g., L. M.Branscomb et al., Phys. Rev. Lett. 98, 1028 (1955)]. The photodetachment cross section is large enough in a broad photon energy range which practically overlaps all visible and near IR spectrums.
Such photons cannot knock out an electron from H0 or all electrons from H− and produce positive ions. This approach was proposed in 1975 by J. H. Fink and A. M. Frank [J. H. Fink et al., Photodetachment of electrons from negative ions in a 200 keV deuterium beam source, Lawrence Livermore Natl. Lab. (1975), UCRL-16844]. Since that time a number of projects for photon neutralizers have been proposed. As a rule, the photon neutralizer projects have been based on an optic resonator similar to Fabri-Perot cells. Such an optic resonator needs mirrors with very high reflectance and a powerful light source with a thin line, and all of the optic elements need to be tuned very precisely. For example, in a scheme considered by Kovari [M. Kovari et al., Fusion Engineering and Design 85 (2010) 745-751], the reflectance of the mirrors is required to be not less than 99.96%, the total laser output power is required to be about 800 kW with output intensity of about 300 W/cm2, and the laser bandwidth is required to be less than 100 Hz. It is unlikely that such parameters could be realized together.
Therefore, it is desirable to provide a non-resonance photo-neutralizer.